The present invention relates to non-acicular modified iron oxides having high coercivity which is used for high density magnetic recording.
Magnetic recording is widely used in computer industries as main frame storage and diskettes, in home appliances as video or audio tapes and so on.
Coventionally, for audio purposes, acicular .tau.-Fe.sub.2 O.sub.3 (maghemite) or Fe.sub.3 O.sub.4 (magnetite) with Hc of 200 to 400 Oe is applicable. For video purposes, Co-.tau.-Fe.sub.2 O.sub.3 with Hc of 600 to 800 Oe is available. However, for high density recording such as high quality and high density tapes and hard diskette, Co-.tau.-Fe.sub.2 O.sub.3 is no longer usable. Iron powder with Hc up to 1500 Oe is usually used. Particulates for next generation applications, for instance, the HDTV purposes, 3.5" and 2.5" ultra-high density diskette etc., seek for even higher coercivity and smaller particle size. Thin film and barium ferrite have been considered. (M. P. Sharrock, MRS Bulletin, Vol. XV, March 1990, P.53)
There are deficiencies for the above mentioned media:
1. For the conventional .tau.-Fe.sub.2 O.sub.3 (including the Co coated ones), besides of being low coercivity, dispersion is a problem due to their acicular shape. Magnetic field alignment is needed due also to their acicular shape.
2. The temperature coefficient of coercivity of Co doped .tau.-Fe.sub.2 O.sub.3 is very high. A. Eiling reported 0.46%/.degree. C. for .tau.-Fe.sub.2 O.sub.3 surface-doped with Co (Hc=650 Oe) and 0.71%/.degree. C. for body-doped ones (Hc=700 Oe) [IEEE Trans. Magn., MAG-23, No. 1, p.16 (1987)], while M. P. Sharrock reported 1.00%/.degree. C. for Co body doped .tau.-Fe.sub.2 O.sub.3 and 0.2 to 0.5%/.degree. C. for surface-doped ones [IEEE Trans. Magn., MAG-25, p.4374 (1989)].
3. For iron particle, the dispersion is even more serious than acicular oxides. It also has the problem of being easily oxidized or corroded.
4. For thin film media, sophisticated facility is required and surface coating is crucial.
5. For barium ferrites, the coercivity tends to be reduced after coating and curing. Besides, they are specifically applicable in perpendicular recording system.
In accordance with the cobalt containing iron oxide, U.S. Pat. No. 4,200,680 discloses a magnetic iron oxide prepared by coating the surface of needle-like particles of magnetic iron oxide with a cobalt compound, U.S. Pat. No. 4,179,387 discloses a process for preparing magnetic iron oxide containing Co which comprises dehydrating and reducing goethite containing Co, and oxidizing gradually the magnetite at a relatively low temperature of lower than about 100.degree. C., in which the oxidation degree is about 30 to 80%. Another invention patent, U.S. Pat. No. 4,015,030, discloses a process for stabilizing a ferromagnetic material comprising heat-treating a needle-like ferromagnetic material.
There was a report presenting the preparation of modified maghemite by adding 2 to 5 wt % cobalt and 5 to 23 wt % manganese into the iron oxide using a special oxalic precursor and precipitated in alcoholic solvent followed by heat treatment [Ph. Tailhades et. al., IEEE Trans. Magn., MAG-26, 1822 (1990)]. The resultant acicular particulate (200 nm, aspect ratio 3) has a maximum coercivity of up to 1800 Oe with a residual magnetization value of 40 to 42 emu/g. However, no saturation magnetization value is shown.
The present invention is different from this report in that (1) much higher Co content is used, (2) non-acicular particulates are resulted, (3) no special precursor is required and aqueous solutions are available.